Introduction What factors, including environmental variables, affect the rate of transpiration in plants? Background Cells and organisms must exchange matter with the environment to grow, reproduce, and maintain organization, and the availability of resource influences responses and activities. For example, water and macronutrients are used to synthesize new molecules, and, in plants, water is essential for photosynthesis. Organisms have evolved various mechanisms for accumulating sufficient quantities of water, ions, and other nutrients and for keeping them properly balanced to maintain homeostasis. Plants absorb and transport water, nutrients, and ions from the surrounding soil via osmosis, diffusion, and active transport. Once water and dissolved nutrients have entered the root xylem, they are transported upward to the stems and leaves as part of the process of transpiration, with a subsequent loss of water due to evaporation from the leaf surface. Too much water loss can be detrimental to plants; they can wilt and die. The transport of water upward from roots to shoots in the xylem is governed by differences in water (or osmotic) potential, with water molecules moving from an area of high water potential (higher free energy, more water) to an area of low water potential (lower free energy, less water). (You will study the concept of water potential in more detail when exploring the processes of osmosis and diffusion in Investigation 4.) The movement of water through a plant is facilitated by osmosis, root pressure, and the physical and chemical properties of water. Transpiration creates a lower osmotic potential in the leaf, and the TACT (transpiration, adhesion, cohesion, and tension) mechanism describes the forces that move water and dissolved nutrients up the xylem. During transpiration, water evaporating from the spaces within leaves escapes through small pores called stomata. Although evaporation of water through open stomata is a major route of water loss in plants, the stomata must open to allow for the entry of CO 2 used in photosynthesis. In addition, O 2 produced in photosynthesis exits through open stomata. Consequently, a balance must be maintained between the transport of CO 2 and O 2 and the loss of water. Specialized cells called guard cells help regulate the opening and closing of stomata. Purpose of Lab In this laboratory investigation, you will begin by calculating leaf surface area and then determine the average number of stomata per square millimeter. From your data, several questions emerge about the process of transpiration in plants. Answer these questions in your lab book. Use complete sentences that restate the question. 1. Do all plants have stomata? Is there any relationship between the number of stomata and the environment in which the plant species evolved? 2. Are leaf surface area and the number of stomata related to the rate of transpiration? What might happen to the rate of transpiration if the number of leaves or the size of leaves is reduced? 3. Do all parts of a plant transpire? 4. Do all plants transpire at the same rate? Is there a relationship between the habitat in which plants evolved and their rate of transpiration? 5. What other factors, including environmental variables, might contribute to the rate of transpiration? 6. What structural features and/or physiological processes help plants regulate the amount of water lost through transpiration? How do plants maintain the balance between the transport of CO 2 and O 2 and the amount of water lost through transpiration? You will then design an experiment to investigate one of these questions or a question of your own. As a supplemental activity, you can examine microscopically thin sections of stems, identify xylem and phloem cells, and relate the function of these vascular tissues to observations made about the structure of these cells. Learning Objectives To investigate the relationship among leaf surface area, number of stomata, and the rate of transpiration
To design and conduct an experiment to explore other factors, including different environmental variables, on the rate of transpiration To investigate the relationship between the structure of vascular tissues (xylem and phloem) and their functions in transporting water and nutrients in plants.general Safety Precautions When appropriate, you should wear goggles for conducting investigation. Nail polish used in the investigation is toxic by ingestion and inhalation, and you should avoid eye contact. All materials should be disposed of properly as per you teacher s instructions. The Investigations Getting Started Questions Answer these in your lab book. Use complete sentences that restate the question. These questions are designed to help you understand concepts related to transpiration in plants before you design and conduct your investigations. 7. If a plant cell has lower water potential than its surrounding environment, make a prediction about the movement of water across the cell membrane. In other words, will the cell gain water or lose water? Explain your answer in the form of a diagram with annotations. 8. In the winter, salt is sometimes spread over icy roads. In the spring, after the ice has melted, grass often dies near these roads. What causes this to happen? Explain your answer in the form of a diagram with annotations. 9. How can you calculate the total leaf surface area expressed in cm 2? In mm 2? 10. How can you estimate the leaf surface area of the entire plant without measuring every leaf? 11. What predictions and/or hypotheses can you make about the number of stomata per mm 2 and the rate of transpiration? 12. Is the leaf surface area directly related to the rate of transpiration? 13. What predictions can you make about the rate of transpiration in plants with smaller or fewer leaves? 14. Because most leaves have two sides, do you think you have to double your calculations to obtain the surface area of one leaf? Why or why not? 15. Water is transpired through stomata, but carbon dioxide also must pass through stomata into a leaf for photosynthesis to occur. There is evidence that the level of carbon dioxide in the atmosphere has not always been the same over the history of life on Earth. Explain how the presence of a higher or lower concentration of atmospheric carbon dioxide would impact the evolution of stomata density in plants. 16. Based on the data in the following table, is there a relationship between the habitat (in terms of moisture) to which the plants are adapted and the density of stomata in their leaves? What evidence from the data supports your answer? (Look up each plant and its habitat. This is an actual AP test question and you have to predict the habitat of these plants based on their stomata density and location.) Table 1. Average Number of Stomata per Square Millimeter of Leaf Surface Area Plant In Upper Epidermis In Lower Epidermis Anacharis 0 0 Coleus 0 141 Black Walnut 0 160 Kidney Bean 40 176 Nasturtium 0 130 Sunflower 85 156 Oats 25 23 Corn 70 88 Tomato 12 130 Water Lily 460 0
Part 1: Determining Number of Stomata per Square Millimeter of Leaf Surface Area Materials Living representative plant species Calculator, microscope, microscope slides, clear cellophane tape, clear nail polish, scissors, transparent ruler Procedure Make a dry mount of a nail polish stomatal peel to view leaf epidermis using the following technique: a. Obtain a leaf. (The leaf may remain on the plant or be removed.) b. Paint a solid patch of clear nail polish on the leaf surface being studied. Make a patch of at least one square centimeter. c. Allow the nail polish to dry completely. d. Press a piece of clean, clear cellophane tape to the dried nail polish patch. Using clear (not opaque) tape is essential here. e. Gently peel the nail polish patch from the leaf by pulling a corner of the tape and peeling the nail polish off the leaf. This is the leaf impression that you will examine. (Make only one leaf impression on each side of the leaf, especially if the leaf is going to be left on a live plant.) f. Tape the peeled impression to a clean microscope slide. Use scissors to trim away any excess tape. Label the slide as appropriate for the specimen being examined and label the side of leaf from which the peel was taken. g. Examine the leaf impression under a light microscope to at least 400X (or highest magnification). Draw and label what you observe. Can you observe any stomata? Search for areas where there are numerous stomata. h. Count all the stomata in one microscopic field. Record the number. i. Repeat counts for at least three other distinct microscopic fields and record the number of stomata. j. Determine an average number of stomata per microscopic field. k. From the average number per microscopic field, calculate the number of stomata per 1 mm 2. You can estimate the area of the field of view by placing a transparent plastic ruler along its diameter, measuring the field s diameter, and then calculating area by using πr 2. (Most lowpower fields have a diameter between 1.5 and 2.0 mm.) l. Repeat steps b - k for the top and the bottom of the leaf. m. Record the stomata density for the top and bottom of the leaf in your lab book. Part 2: Designing and Conducting Your Investigation Using Potometer Method for Determining Rate of Transpiration Material Plant species available in your region/season such as chrysanthemums Potometer, which you assemble from clear plastic tubing, a ring stand with clamp, a pipette A container for water Fan, heat lamp, small plastic bag, spray bottle with water, salt or other solutions Petroleum jelly to make an airtight seal between the cut end of the stem and tubing filled with water. String or tape Scissors Timer Experiment Each lab group will conduct the experiment in a different environment. Possible environments are normal conditions, high humidity, high wind, and high heat. Hypothesis! Make a hypothesis/prediction about which environmental factors will have the greatest affect on transpiration rates. Be sure to explain you hypothesis. Procedure
Step 1 Take a cutting of the plant that contains several leaves. Remove the flowers from the cutting. Calculate the total surface area of the leaves on your cutting in cm 2 and record the value. Step 2 Use scissors to carefully cut the plant stem at an angle so its diameter fits into the piece of plastic tubing in the potometer assembly. *Note that it is often helpful to cut the stem while it is submerged under water to prevent air bubbles from begin introduced into the xylem. Your teacher will provide additional instructions, if necessary. Step 3 Set up your environmental condition. Step 4 Set up the cutting in the potometer. See additional instructions. Step 5 Wait 5-10 minutes to allow the system to reach equilibrium. Record the rate at which the water level drops after 30 minutes. Step 6 Calculate the rate of transpiration/surface area and record this value. Step 7 Put all the data on the board and record your classmates data in a table. Step 8 Graph all the data. Conclusion Analyzing Results 1. How was the rate of transpiration affected by your choice of experimental variable as compared to the control? By comparing results and conclusions, explain how changes or variables in environmental conditions affect transpiration rates. 2. Based on data collected from different lab groups, which environmental variable(s) resulted in the greatest rate of water loss through transpiration? Explain why this factor might increase water loss when compared to other factors? 3. Why did you need to calculate leaf surface area to determine the rate(s) of transpiration? 4. What structural or physiological adaptations enable plants to control water loss? How might each adaptation affect transpiration? 5. Make a prediction about the number of stomata in a leaf and the rate of transpiration. What type(s) of experiments could you conduct to determine the relationship between the number of stomata and the rate of transpiration? 6. Create a diagram with annotation to explain how the TACT (transpiration, adhesion, cohesion, tension) mechanism enables water and nutrients to travel up a 100-ft. tree. Predict how a significant increase in ambient (environmental) temperature might affect the rate of transpiration in this tree. Explain your prediction in terms of TACT and the role of guard cells in regulating the opening and closing of stomata. http://home.earthlink.net/~dayvdanls/plant_transport.html Evaluating Results 1. Was your initial hypothesis about the effect of your environmental variable on the rate of transpiration supported by the data you collected? Why or why not? 2. What were some challenges you had in performing your experiment? Did you make any incorrect assumptions about the effect of environmental variables on the rate(s) of transpiration? 3. If you had to revise the design of your experiment, what suggestions would you make? Why would you make them? 4. What further experiments could you perform using these procedures to test transpiration rates?
Write Up Rubric Title, Date Partners 1 Objectives/Hypotheses 3 Getting Started Questions 5 Procedure Summary Part 1 and 2 4 Data/Graph Part 1 and 2 4 Calculations 3 Analysis Questions 6 Evaluating Results Questions 4 Total Points 30